Treatment of an ultrafiltrate derived from an electrodeposition process by reverse osmosis

ABSTRACT

THIS INVENTION RELATES TO THE USE OF REVERSE OSMOSIS TO TREAT ULTRAFILTRATE DERIVED FROM AN ELECTRODEPOSITION PROCESS IN ORDER TO PROVIDE A RELATIVELY MORE PURE PROCESS EFFLUENT AND/OR TO PROVIDE RINSE WATER TO THE PROCESS OF RELATIVELY HIGHER PURITY.

May 16, 1972 oop EI'AL 7 3,663,407

TkEA'l'MEN'l' OF AN ULTRAFILTRA'IE DERIVED EMJIA AN [#MQCLRODEPOSITION PROCESS BY REVERSE OSMOSIS Filed June 121, 1971 2 Sheets-Sheet 1 INVENTORJ fR Dt'RI K M M P nun/n 6. 30540 2774 BY M 4 Z'M- ATTORNEYS United States Patent lice 3,663,407 TREATMENT OF AN ULTRAFILTRATE DERIVED FROM AN ELECTRODEPOSITION PROCESS BY REVERSE OSMOSIS Frederick M. Loop, North Olmsted, and Frank C. Bosworth, Bay Village, Ohio, assiguors to PPG Industries, Inc., Pittsburgh, Pa.

Filed June 21, 1971, Ser. No. 155,042 Int. Cl. B01k 5/02; C23b 13/00 US. Cl. 204-181 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the use of reverse osmosis to treat ultrafiltrate derived from an electrodeposition process in order to provide a relatively more pure process efiluent and/or to provide rinse water to the process of relatively higher purity.

STATE OF THE ART Electrodeposition has become a widely commerciallyaccepted industiral coating technique. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run or wash ofi during baking. Virtually any conductive substrate may be coated by electrodeposition. Most commonly employed are metal substrates including metals such as iron, steel, copper, zinc, brass, tin, nickel, chromium and aluminum, as well as other metals and pretreated metals. lmpregnated paper or other substances rendered conductive under the conditions of the coating process may also be employed as substrates.

In the electrodeposition process, the articles to be electrocoated are immersed in an aqueous dispersion of a solubilized, ionized, film-forming material such as a synthetic organic vehicle resin. An electric current is passed between the article to be coated, serving as an electrode, and a counter-electrode to cause deposition of a coating of the vehicle resin on the articles. The articles are then withdrawn from the bath, usually rinsed, and then the coating either air-dried or baked in the manner of a conventional finish.

In a continuous electrodeposition process, several process parameters among others have presented substantial problems. These include control of the electrodeposition bath composition, dragout losses and the disposal problems of rinse water containing dragout.

The application of ultrafiltration to an electrodeposition process has allowed improvements with regard to each of these parameters. The use of ultrafiltra-tion for bath control and dragout conservation are described in application Ser. No. 814,789, filed Apr. 9, 1969 and in application Ser. No. 881,259, filed Dec. 1, 1969, now abandoned. The application of ultrafiltration to an electrodeposition process had produced numerous benefits.

Ultrafiltration of an electrodepositable composition selectively removes low molecular weight materials from the bath composition. This selective filtration process removes excess counter-ion and thus serves as a method of conventional bath control, but in addition this method further removes other excess materials or contaminants from the bath, thus permitting more complete control of the baths constituents than has heretofore been possible. The ultrafiltrate, that is, the efiluent from the process, being essentially vehicle resin free, may be employed as rinse water in a manner so that the rinse water returns to the electrodeposition bath, returning dragout solids previously lost to the electrodeposition bath without adding substantial amounts of additional water to the system, thus overflowing the electrodeposition bath.

3,663,407 Patented May 16, 1972 It has been found that while ultrafiltrate is substantially purer aqueous media than the electrodeposition bath in that it is substantially free of high molecular weight resinous materials and pigments and the like, it does contain anionic, cationic and non-ionic materials from the paint in most instances in a ratio proportional to their concentration in the water phase of the paint. Thus, for example, ultrafiltrate, depending on the composition of the electrodeposition bath, can possibly contain amines, alkali metal ions, phosphates, chromates, sulfates, solvents, salt, and carbon dioxide, as well as other constituents.

It has now been found that ultrafiltrate can be subjected to reverse osmosis separation, providing a concentrate or retentate containing a substantial portion of the anionic, cationic and non-ionic materials in the ultrafiltrate, while passing a substantially purer aqueous efiluent. This allows for substantial refinements and improvements in the electrodeposition process. -First, where ultrafiltrate has previously been sent to drain as a method of bath control, a waste disposal problem still exists albeit less serious since while passing ultrafiltrate to drain rather than paint or drag-out containing rinse, the ultrafiltrate still contains components which may be deemed deleterious to the ecology. Reverse osmosis concentrates the deleterious materials for disposal by other means while passing a substantially purer effluent to drain. Secondly, where ultrafiltrate is employed as rinse water, frequently the system must be run open, passing ultrafiltrate to drain for bath control, thus requiring the addition of alternative sources of water such as deionized water for rinsing. Reverse osmosis of ultrafiltrate, on the other hand, provides a more continuous supply of aqueous media for rinsing of substantially higher purity than ultrafiltrate while removing contaminants or deleterious materials in the concentrate or retentate. This has an added advantage in that the risk of water spotting on the finished article is substantially reduced in that water spotting is directly related to the amount of impurities contained in the water drops remaining upon the article after rinsing. Thus, where the rinse water is of substantial purity, such as deionized water, water spotting does not present a substantial problem. Likewise, where the efiluent of the reverse osmosis treatment of ultrafiltrate is employed as rinse water, substantially tewer water spotting problems are encountered under the same conditions as when ultrafiltrate is employed as rinse water.

The use of the technique herein described is highly flexible. Where rinsing employing the efiluent from reverse osmosis treatment is employed, it may be accomplished in such a way that the effluent and drag-out are either collected separate from the bath or immediately returned to the bath. Likewise, since the process of the invention is a method of bath control, it may be desirable in order to maintain or change the composition of the bath to intermittently or continuously remove at least a portion of the efiluent of either the ultrafiltration process or the subsequent reverse osmosis process of the system. Thus, the rinsing of drag-out may be intermittently conducted with tap water or preferably deionized water, either separate from the bath or in the manner so that this rinse returns to the bath. Likewise, a mixture of reverse osmosis eflluent and other water or even ultrafiltrate may be employed as rinsing media.

In the attached drawing, an apparatus used to carry out the method of this invention is schematically illustrated. 'Electrodeposition bath 1 from which films are deposited uses suitable apparatus (not shown). A portion of the bath may be continuously or intermittently withdrawn through an outlet in valve 2 and passed through line 3 to an ultrafilter 4. Here, in the ultrafiltration process water, free counter ions and counter ions present as low molecular weight salts, for example, carbonate, as Well as other low molecular Weight species, if present, are separated from the vehicle resin through ultrafiltration membrane 19. The concentrate or retentate comprises an aqueous dispersion of vehicle resin, pigment and other high molecular weight components of the bath composition. The ultrafiltrate is removed from the ultrafilter passing through line 5 through the use of valve 6 while the concentrate or retentate is returned to the bath through line 15 and valve 16. The ultrafiltrate may be directed either unidirectionally or proportionally in either an intermittent or continuous fashion to drain 7 or to a reverse osmosis unit 21. Here, in the reverse osmosis process, water, free counter ions, counter ions present as low molecular weight salts, for example, carbonate, as well as other low molecular weight species, are retained in the concentrate or retentate while there passes through the reverse osmosis membrane substantially purer aqueous media than enters the reverse osmosis unit through line 20. The concentrate or retentate may be passed to drain through line 23 through the use of valve 28 or may be returned to the electrodeposition bath through line 29, or even through the use of appropriate piping may be used as a first stage rinse. All the efi luent from the reverse osmosis process is passed through valve 27 and line 24, either unidirectionally or proportionately in either intermittent or continuous fashion to drain, for use as rinse material or for direct return to the electrodeposition bath. In the latter two cases, the efliuent from the reverse osmosis process is passed from line 24 through valve 26 to line 8, where it is directed through valves 9 to 13 to either a rinse station 14 for rinsing drag-out in a manner that drag-out containing rinse returns to the bath or through 10 for rinsing drag-out in a manner so that it does not return directly to the bath. Valves 9 and 13 likewise accommodate the intermittent or proportional use of Water 11 or 12 rather than reverse osmosis eflluent for rinsing. Likewise, valves 6 and 26 allow for the proportional or intermittent use of ultrafiltrate for rinsing. Line 17 allows for the return of ultrafiltrate or reverse osmosis effiuent to the bath if and when desired. As stated, this drawing is schematic and does not purport to describe the necessary pumping means or apparatus which are well-known in the art.

As previously stated, the control of an electrodeposition bath by an ultrafiltration process has been described in copending application Ser. No. 814,789, filed Apr. 9, 1969. Ultrafiltration is a process which separates materials below a given molecular Weight size from the high molecular weight components in an electrodeposition bath. With properly selected membranes, this treatment does not remove any product or desirable resin from the paint or the tank. It does remove low molecular weight anionic, cationic and non-ionic materials from the paint in a ratio to their concentration in the water-phase of the paint.

lUltrafiltration may be defined as a method of concentrating solute while removing solvent, or selectively removing solvent and low-molecular weight solute from a significantly higher molecular weight solute. From another aspect, it is a process of separation whereby a solution containing a solute of molecular dimensions significantly greater than the solvent is depleted of solute by being forced under a hydraulic pressure gradient to flow through a suitable membrane. The first definition is the one which most fittingly describes the term ultrafiltration as applied to an electrodeposition bath.

Ultrafiltration thus encompasses all membrane-moderated, pressure-activated separations wherein solvent or solvent and smaller molecules are separated from modest molecular weight macromolecules and colloids. The term ultrafiltration is generally broadly limited to describing separations involving solutes of molecular dimensions greater than about ten solvent molecular diameters and below the limit of resolution of the optical microscope, that is, about 0.5 micron. In the present process, water is considered the solvent.

The principles of ultrafiltration and filters are discussed in a chapter entitled Ultrafiltration in the Spring, 1968, volume of Advances in Separations and Purifications, E. S. Perry, editor, John Wiley & Sons, New York, as Well as in Chemical Engineering Progress, volume 64, December 1968, pages 31 through 43, which are hereby incorporated by reference.

The basic ultrafiltration process is relatively simple. Solution to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in a cell, in contact with an appropriate filtration membrane supported on a porous support. Any membrane or filter having chemical integrity to the system being separated and having the desired separation characteristic may be employed. Preferably, the contents of the cell should be subjected to at least moderate agitation to avoid accumulation of the retained solute on the membrane surface with the attendant binding of the membrane. Ultrafiltrate is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent or solvent plus dissolved low molecular weight solute is removed. A suitable apparatus for conducting ultrafiltration is described in U.S. Pat. No. 3,494,465, which is hereby incorporated by reference.

There are two types of ultrafiltration membrane. One is the microporous ultrafilter, which is a filter in the traditional sense, that is, a rigid, highly-voided structure containing interconnected random pores of extremely small average size. Through such a structure, solvent (in the case of electrodeposition, water) zflows essentially viscously under a hydraulic pressure gradient, the flow rate proportional to the pressure difierence, dissolved solutes, to the extent that their hydrated molecule dimensions are smaller than the smallest pores within the structure, Will pass through, little impeded by the matrix. Larger size molecules, on the other hand, will become trapped therein or upon the external surface of the membrane and will thereby be retained. Since the microporous ultrafilters are inherently susceptible to internal plugging or fouling by solute molecules whose dimensions lie within the pore size distribution of the filter, it is preferred to employ for a specific solute a microporous ultrafilter whose mean pore size is significantly smaller than the dimensions of the solute particle being retained.

In contrast, the difiusive ultrafilter is a gel membrane through which both solvent and solutes are transported by molecular ditfusion under the action of a concentration or activity gradient. In such a structure, solute and solvent migration occurs via random thermal movements of molecules within and between the chain segments comprising the polymer network. Membranes prepared from highly hydrophilic polymers which swell to eliminate standard water are the most useful diffusive aqueous ultrafilter membranes. Since a diffusive ultrafilter contains no pores in the conventional sense and since concentration within the membrane of any solute retained by the membrane is low and time-independent, such a filter is not plugged by retained solute, that is, there is no decline in solvent permeability with time at a constant pressure. This property is particularly important for a continuous concentration or separation operation. Both types of filters are known in the art.

The presently preferred ultrafilter is an anisotropic membrane structure such as illustrated in FIG. 1. This structure consists of an extremely thin, about one-tenth to about ten micron layer, of a homogeneous polymer 1 supported upon a thicker layer of a microporous opencelled sponge 2, that is, a layer of about 20 microns to about 1 millimeter, although this dimension is not critical. If desired, this membrane can be further supported by a: fibrous sheet, for example, paper, to provide greater strength and durability. These membranes are used with a thin film or skin exposed to the high-pressure solution. The support provided to the skin by the spongy substrate is adequate to prevent film rupture.

Membranes useful in the process are items of commerce and can be obtained by several methods. One general method is described in Belgian Pat. No. 721,058. This patent describes a process which in summary comprises (a) forming a casting dope of the polymer in an organic solvent, (b) forming a film of the casting dope, and (c) preferentially contacting one side of said film with a diluent having high compatibility with the casting dope to efiect precipitation of the polymer immediately upon coating the cast film with the diluent.

The choice of a specific chemical composition for the membrane is determined to a large extent by its resistance to the chemical environment. Membranes can be typically prepared from thermoplastic polymers such as polyvinyl chloride, polyacrylonitrile, polysulfones, poly(methyl methacrylate), polycarbonates, poly(n-butyl methacrylate), nylons, as Well as a large group of copolymers formed from any of the monomeric units of the above polymers, including Polymer 360, a polysulfone copolymer. Cellulose materials such as cellulose acetate may also be employed as membrane polymers.

Same examples of specific anisotropic membranes operable in the process of the inevntion include: Diafiow membrane ultrafilter PM-30, the membrane chemical composition of which is polysulfone copolymer, Polymer 360, and which has the following permeability characteristics:

Solute Retention Characteristics The membrane is chemically-resistant to acids (HCl, H 50 H PO all concentrates), alkalis, high phosphate buffer and solutions of common salts as well as concentrated urea and guanadine hydrochloride. The membrane is solvent-resistant to alcohol, acetone and dioxane. The membrane is not solvent-resistant to dimethylformamide or dimethylsulfoxide. This membrane is hereinafter referred to as Membrane A.

Dorr-Oliver XPA membrane, the membrane chemical composition of which is Dynel (an acrylonitrile-vinyl chloride copolymer) and which has the following permeability characteristics:

Molecular Percent Solute weight retention Flux Cytochrome C 12, 600 50 100 a- Chymotripsinagen- 24, 000 90 22 Ovalbumin. 45, 000 100 45 1 GaL/sq. ftJday at 30 p.s.i., 1.0% solute.

This membrane is hereinafter referred to as Membrane B.

Dorr-Oliver BPA membrane, the membrane chemical composition of which is phenoxy resin (polyhydroxy ether) and which has the following permeability characteristics:

1 Gal./sq.tt./day at 30 p.s.i., 1.0% solute.

This membrane is hereinfater referred to as Membrane C.

The microporous ultrafilters are generally isotropic structures, thus flow and retention properties are inde pendent of flow direction. It is preferred to use an ultrafilter which is anisotropic in its microporous membrane structure, FIG. 2. In such a membrane, the pore size increases rapidly from one face to the other. When the fine textured side 4 is used in contact with the feed solution, this filter is less susceptible to plugging since a particle which penetrates the topmost layer cannot become trapped in the membrane because of the large pore size 5 in the substrate.

The process of the invention may be operated as either a batch or a continuous process. In batch selective filtration or batch ultrafiltration, a finite amount of material is placed in a cell which is pressurized. A solvent and lower molecular weight solutes are passed through the membrane. Agitation is provided by a stirrer, for example, a magnetic stirrer. Obviously, this system is best used for small batches of material. In a process requiring continuous separation, a continuous selective filtration process is preferred. Using this technique, material is continuously recirculated under pressure against a membrane or series of membranes through interconnecting flow channels, for example, spiral flow channels.

Likewise, the ultrafiltration process may be conducted as either a concentration process or a diafiltration process. Concentration involves removing solvent and low molecular weight solute from an increasingly concentrated retentate. Filtration flow rate will decrease as the viscosity of the concentrate increases. Diafiltration, on the other hand, is a constant volume process whereby the starting material is connected to a reservoir of pure solvent, both of which are placed under pressure simultaneously. Once filtration begins, the pressure source is shut off in the filtration cell and, thus, as the filtrate is removed, an equal volume of new solvent is introduced into the filtration cell to maintain the pressure balance.

The configuration of the filter may vary widely and is not limiting to the operation of the process. The filter or membrane may, for example, be in the form of sheets, tubes or hollow fiber bundles, among other configurations.

Under ideal conditions, selected low molecular weight solutes would be filtered as readily as solvent and their concentration in the filtrate' is equal to that in the retentate. Thus, for example, if a material is concentrated to equal volumes of filtrate and retentate, the concentration of low molecular weight solute in each would be the same.

.Using diafiltration, retentate solute cencentration is not constant and the mathematical relationship is as follows:

io a where C is the initial solute concentrate, C, is the final solute concentration of the retentate, V is the volume of solute delivered to the cell (or the volume of the filtrate collected) and V is the initial solution volume (which remains constant).

Electrodepositable compositions, while referred to as solubilized, in fact are considered a complex solution, dispersion or suspension or combination of one or more of these classes, in water, which acts as an electrolyte under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances and perhaps in most the vehicle resin is a dispersion which may be called a molecular dispersion of molecular size between a colloidal suspension and a true solution.

The typical industrial electrodepositable composition also contains pigments, crosslinking resins and other adjuvants which are frequently combined with the vehicle resin in a chemical and physical relationship. For example, the pigments are usually ground in a resin medium and are thus wetted with the vehicle resin. As can be readily appreciated then, an electrodepositable composition is complex in terms of the freedom or availability with respect to removal of a component or in terms of the apparent molecular size of a given vehicle component.

As applied to the process of this invention, ultra-filtration comprises subjecting an electrodepositable composition, especially after it has been employed in a coating process, which inherently causes contaminants and other low molecular weight materials to accumulate in the bath, such as metal pretreatment chemicals, water, absorbed CO (either dissolved, or, more likely, combined as an aminic salt or carbonate), neutralizing agent, organic solvent and ions such as chromate, phosphate, chloride and sulfate, for example, to an ultrafiltration process employing an ultrafilter, preferably a diffusive membrane ultrafilter selected to retain the solubilized vehicle resin while passing water and low molecular weight solute, resin While passing water and low molecular weight solute, especially those with passing water and low molecular weight solute, especially those with a molecular weight below about 500. As previously indicated, the filters discriminate as to molecular size rather than actual molecular weight, thus, these molecule weights merely establish an order of magnitude rather than a distinct molecular weight cut-01f. Likewise, as previously indicated, the retained solutes may, in fact, be colloidal dispersions or molecular dispersions rather than true solutes.

In practice, a portion of the electrodepositable composition may be continuously or intermittently removed from the electrodeposition bath and passed under pressure created by a pressurized gas or by means of pressure applied to the contained fluid in contact with the ultrafilter. Obviously, if desired, the egress side of the filter may be maintained at a reduced pressure to create the pressure difference.

The pressures necessary are not severe. The maximum pressure, in part, depends on the strength of the filter. The minimum pressure is that pressure required to force water and low molecular weight solute through the filter at a measurable rate. With the presently preferred membranes, the operating pressures are between about and 150 p.s.i., preferably between about 25 and 75 p.s.i. Under most circumstances, the ultrafilter should have an initial flux rate, measured with the composition to be treated of at least about 3 gaL/ftfi/day (24 hours) and preferably at least about 4.5 gal./ft. /day.

As previously indicated, the bath composition should be in motion at the face of the filter to prevent the retained solute from impeding the flow through the filter. This may be accomplished by mechanized stirring or by fluid flow with a force vector parallel to the filter surface.

The retained solutes comprising the vehicle resin are then returned to the electrodeposition bath. If desired, the concentrate may be reconstituted by the addition of water either before entry to the bath or by adding water directly to the bath.

If there are present in the bath desirable materials which, because of their molecular size, are removed in the ultrafiltration process, these may likewise be returned to the bath either directly to the retained solute before entry to the bath, in the makeup feed as required, or independently.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable compositions utilized in the practice of this invention. Virtually any water-soluble, water-dispersible or water-emulsifiable polyacid or polybasic resinous material can be electrocoated and, if film-forming, provides coatings which may be suitable for certain purposes. Any of such electrodepositable compositions is included among those which may be employed in the present invention, even though the coating obtained may not be entirely satisfactory for certain specialized uses. Electrodepositable compositions, while referred to as solubilized, in fact are considered complex solutions, dispersions or suspensions, or a combination of one or more of these classes in water, which acts as an electrolyte under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances, and perhaps in most, the vehicle resin is a dispersion which may be called a molecular dispersion of molecular size between a colloidal suspension and a true solution. Numerous such resins are described in U.S. Pats. Nos. 3,230,162; 3,441,489; 3,422,044; 3,403,088; 3,369,983; 3,366,563; 3,3 82,165 and British Pat. No. 1,132,267 as well as other patents to be found in Class 204-, sub-class 181, of the US. Patent Oflice. Since these materials are a wellknown, art-recognized class of materials, it is deemed unnecessary to set forth a description of those resins in detail. The resin and electrodepositable composition descriptions of the above-mentioned patents are hereby incorporated by reference. For a general review of electrodeposition paint formulation, reference may be had to R. L. Yeates, Electropainting, Robert Draper, Ltd., Teddington, England (1966).

Presently the most widely used electrodeposition vehicle resins are synthetic polycarboxylic acid resinous materials; however, polyacids other than polycarboxylic acids are known in the art as electrodepositable resins. Likewise, polybasic resins may be employed.

The polyacids are anionic in nature and are dispersed or dissolved in water with alkaline metals such as amine or alkaline metal hydroxides and when subjected to electric current they migrate to the anode. Polybasic resins solubilized by acids are cationic in nature and when these resins are Water-dispersed or solubilized with an acid, the material is deposited on the cathode under an electric current. Although most electrodepositable compositions are a complex mixture, most commercially-utilized electrodepositable compositions are a complex mixture of either the anionic or cationic resins described above formulated with adjuvants such as pigments, solvents and surfactants, crosslinking resins and the like.

As previously stated, the ultrafiltrate is subjected to reverse osmosis. Ultrafiltration can be deemed a separation process that is separating large molecules from smaller molecules. Reverse osmosis, on the other hand, is a purification process, that is, a process to produce a substantially purer aqueous effluent. Osmosis may be described as the migration of pure water through a semipermeable membrane spontaneously into a concentrated solution until equilibrium. The head developed at equilibrium is defined as osmotic pressure. Reverse osmosis involves applying a pressure greater than the osmotic pressure to cause water to flow in a reverse manner through the semi-permeable membrane. Stated otherwise, a pressure exceeding the osmotic pressure is applied to the concentrated solution to cause flow from the concentrated side to the pure side. Typical operating pressures are at least several hundred p.s.i., but always greater than the osmotic pressure of the system.

Reverse osmosis membranes and the technology of their preparation are well-known in the art.

The choice of a specific chemical composition for the membrane is determined to a large extent by its resistance to the chemical environment. Membranes can be typically prepared from thermoplastic polymers such as polyvinyl chloride, polyacrylonitrile, polysulfones, poly(methyl inethacrylate), polycarbonates, poly(n-butyl methacrylate), nylons, as well as a large group of copolymers formed from any of the monomeric units of the above EXAMPLE I The electrodepositable composition utilized in this ex ample was prepared as follows:

A pigment paste. was formulated as follows:

PASTE A Parts by weight 20 percent maleinized" oil (total solids content 97.6%) Diethylamine 20 percent maleic anhydrlde, 80 percent linseed 011, male inized oil having a viscosity of 100,000 centipoises.

were mixed 20 minutes in a closed container. There was then added:

Deionized water 32.00 Dispersing agent (combination oil-soluble sulfonate and non-ionic surfactant)Witco 912 1.48 Anthracite coal (pigmentary) 20.00

Basic lead silicate 8.00 Manganese dioxide 2.00 Strontium chromate 2.00

The above components were ground in a conventional zirco mill to a 7% Hegman grind gauge reading.

Paste A was reduced as follows:

COMPOSITION B Parts by weight 395.2 667.4

Paste A Deionized water Potassium hydroxide solution (15 percent in water) The vehicle resin employed in formulating Composition C (below) was comprised of a maleinized tall oil fatty acid ester of a styrene-allyl alcohol copolymer of 1100 molecular weight and a hydroxyl functionality of comprising 38.5 percent of the copolymer, 55.5 percent tall oil fatty acids, and 6.0 percent maleic anhydride as a 100 percent solids vehicle having an intrinsic viscosity Of 120,000 centipoises and an acid number of 40.6.

COMPOSITION C Vehicle resin above at 100 percent solids content 1319.6 Wetting agent (sorbitan monolaurate) 129.6 Hexakis (methoxymethyl)melamine 159.8 Ethyl Cellosolve 67.0 were mixed 10 minutes and there was added: Composition B 1477.4

The composition was again mixed 10 minutes and there was added:

Deionized water 1015.2

The composition was again mixed 10 minutes and there was added:

Composition C had the following characteristics:

Solids content (percent) 45.1 pH 9.2 Pigment-to-binder ratio 0.2:1.0

10 Composition C was reduced to produce Composition D (below):

COMPOSITION D Composition C 2930.0 Deionized water 8070.0

Composition D had the following properties:

PH 9.45 Conductivity, ,umhos/cm, 75 F. 2300 Total solids content (percent) 12.2 Ash 1.15 MEQ grams total 6.40 MEQ /100 grams solids 52.5 Nitrogen content (percent) 0.24 CO (p.p.m.) 125 Ethyl Cellosolve (percent) 0.42

1 Mtlliequivalents of base.

The above composition was utilized to electrocoat articles through numerous turnovers, the bath being controlled by ultrafiltration utilizing Membrane C set forth above at 50 p.s.i. The ultrafiltrate was then subjected to treatment by reverse osmosis utilizing a Permasep Permeator Membrane having a hollow fiber configuration and characterized by the following parameters at 400 p.s.i., 50 percent conversion:

TAB LE I Salt Feed Permeate passage (p .p .m.) (p .p .m (percent) Specific Percent conductivity non- (mieromhos) pH volatiles Filtrate before reverse osmosis 972 8. 43 0. 30 Effiuent of reverse osmosis 10 7. 95 0. 00

Other electrodepositable compositions, such as those hereinabove described, can be substituted for those of the examples Likewise, various ultrafilters, reverse osmosis systems, and method variations may be employed to obtain the improvements hereinabove described.

According to the provisions of the patent statutes, there are described above the invention and what are now considered its best embodiments; however, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically described.

We claim:

1. In a method of operating an electrodeposition process wherein an electrically-conductive substrate is electrocoated from an aqueous electrodeposition bath which comprises ionically solubilized synthetic organic resin, the steps comprising subjecting at least a portion of said electrodeposition bath to an ultrafiltration process wherein the ultrafiltration membrane passes a first aque- 11 ous effluent comprising water and solute of substantially lower molecular size than the solubilized resin, returning retentate from the ultrafiltration process to the electroposition bath, subjecting said first aqueous efliuent to a reverse osmosis process whereby a second, purer aqueous effiuent is produced. 1

2. A method as in claim 1 wherein the ultrafiltration process operates at a pressure gradient of between about 10 and about 150 psi. and the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day. 7

3. A method as in claim 2 wherein the resin is a'base solubilized synthetic polycarboxylic acid resin.

4. A method as in claim 3 wherein the base is an alkali metal hydroxide.

5. A method as in claim 1 wherein at least a portion of said second efiluent is employed as at least a portion of the rinse water in the electrodeposition process.

6. A method as in claim 5 wherein the ultrafiltration a 12 t process operates at a pressure gradient of between about 10 and about 150 p.s.i., and the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.

7. A method as in claim 5 wherein the resin is a basesolubilized synthetic polycarboxylic acid resin.

8. A method as in claim 7 wherein the base is an alkali metal hydroxide.

References Cited UNITED STATES PATENTS 3,526,588 a 9/ 1970 Michaels et al. 2l023 3,556,970 1/1971 Wallace et a1 204-181 HOWARD S. WILLIAMS, Primary Examiner US. Cl. X.R. 

